(1) Field of the Invention
The present invention relates to a method for preparing an original for projection according to the electrophotographic process and also to a transfer film for the electrostatic photography, which is used for carrying out this method. More particularly, the present invention relates to a method for preparing an original for projection, which is excellent in the smoothness and image characteristics.
(2) Description of the Prior Art
The conventional method for preparing a projection original for an overhead projector (transparent sheet) according to the electrostatic photography comprises transferring a toner image formed on a photosensitive plate for the electrophotography by known means, onto a biaxially stretched polyester film, and fixing the transferred toner image on the film by a heat oven or the like. Among various plastic films, a biaxially stretched polyester film is especially excellent in the heat resistance and the dimension stability, but this film is defective in that the film surface is rendered uneven by the heat applied at the step of fixing the toner image and the surface smoothness is lost.
Furthermore, since the electric resistance of such polyester film is too high, when the transfer film is peeled from the photosensitive plate on transfer of the toner image, discharge breakdown is caused between the photosensitive plate and the film and white spots are formed on the solid black portion of the image by this discharge breakdown. This is another defect of the polyester film. Moreover, because of uneven contact between the photosensitive plate and the transfer film and scattering of the toner on peeling of the transfer film from the photosensitive plate, the difference of the transfer efficiency among areas and the disarray of the transferred image, especially in areas of fine lines, are caused and therefore, an original for projection which corresponds precisely to the original to be reproduced cannot be obtained.
It may be considered that these defects will be eliminated by subjecting the above-mentioned polyester film to a treatment for rendering the surface electrically conductive. In case of a one-component type developer described hereinafter, the transfer efficiency is drastically reduced by such conductive treatment, and a defect of drastic reduction of the image density in the original for projection results.
It may also be considered that the above-mentioned undesired loss of the surface smoothness by heat fixation will be overcome by the use of a one-component type magnetic developer for pressure fixation. However, ordinary polyester films have no fixing property to such developer and therefore, disturbance of the image becomes more prominent owing to scattering of the developer particles on contact with a fixing pressure roller.